Sterile milk, or commercially sterile milk may be defined as a product which is free of microorganisms which can grow under the prevailing conditions. A sterile milk, packed under aseptic conditions in aseptic packages, enjoys the advantage of being able to be distributed and stored at room temperature for a lengthy period of time.
In order to produce such a commercially sterile milk, the commonest method hitherto has been UHT treatment (Ultra High Temperature), i.e. treatment of the milk at elevated temperature, normally between 135.degree. and 150.degree.C. for between 4 and 15 seconds. As a result of the heat treatment, microorganisms are destroyed so that the resultant product is a milk of long shelf-life at room temperature. However, the drawback inherent in milk treated by the UHT method is that the high temperature imparts to the milk ac cooked flavour.
With a view to reducing the contents of harmful microorganisms, use has recently been made of microfiltration, a method which is described in Swedish Patent Specification SE 451 791. In the described method, use is made of a microfilter, normally of ceramics, through which the skimmed milk fraction, after separation, is caused to pass. In the microfilter, the skimmed milk is divided into a permeate flow which has a considerably reduced content of microorganisms, and a retentate flow, which contains the majority of bacteria and spores. The retentate is treated at elevated temperature and thereafter returned to the permeate. This method gives a product which has a longer shelf-life than normal, pasteurised milk, but it must be kept under cold storage and cannot be considered as sterile.
In Swedish patent application 9602546-5, use is made of microfiltration to produce an aseptic consumer milk. By employing a microfilter of slight pore size, milk with a higher fat content can be filtered. The permeate flow, which contains the major fraction of the thermosensitive whey proteins, is free of microorganisms. The retentate flow, which contains fat, casein and microorganisms, undergoes a high temperature treatment, whereafter both of the flows are mixed. Employing this method, there will be obtained a commercially sterile product which, however, contains the destroyed microorganisms.
The present invention relates to a method of obtaining, primarily, a commercially sterile skimmed milk for consumption, but by the admixture of the cream fraction in suitable proportions, it is possible, employing this method, to obtain a consumer milk with varying fat content. The present invention also entails a method which gives a sterile product which is as good as free of destroyed microorganisms and which thereby may be considered as purer than milk produced according to prior art methods.